1. Technical Field
This invention relates to a post-processing apparatus provided with a punch processing section capable of forming a punch hole on a paper sheet.
2. Description of Prior Art
In an image forming system in which a post-processing apparatus is connected with an image forming apparatus, the post-processing apparatus is desired to be capable of conducting various post-processing. As a kind of post-processing apparatus, a post-processing apparatus to form punch holes for filing is available.
In this kind of post-processing apparatus, there are two types of punch processing methods: (1) a batch method where the punch processing is correctively conducted after image formed paper sheets are bundled, (2) a sequential method where the punching process is conducted on each one paper sheet, and paper sheets formed with punch holes are bundled after being conveyed downstream. In the sequential method, a punch processing section is arranged on a conveyance path, and the punch processing is executed after the paper sheet is temporally stopped on the conveyance path, then, upon finishing the punch processing the paper sheet is again conveyed downstream (please refer to Unexamined Japanese Patent Application Publication No. 2004-261951: JP 2004-261951A).
In the batch method, maximum thickness of paper sheets (number of paper sheet of the bundled paper sheets) capable of punch processing is determined by an ability of the punch processing section, however, in the sequential method in many cases said maximum thickness does not mattered since the punch processing is conducted by each one paper sheet.
Meanwhile, in cases where an image forming system capable of various post-processing is utilized in a field of light printing (or in-plant printing), increased number of paper sheet capable of processing per unit time period (hereinafter referred as productivity) is desired. Further, in many cases this productivity is determined by the ability of the post-processing apparatus rather than the image forming apparatus.
In the post-processing apparatus, since the paper sheet is usually stopped temporally for processing, it is necessary to ensure a sufficient interval distance between sequentially conveyed paper sheets (hereinafter referred as paper sheet interval). As a means for the above, it is practiced to increase the paper sheet conveyance speed in the post-processing apparatus faster than the paper sheet conveyance speed in the image forming apparatus. However, in the high speeding trend of recent years, since the conveyance speed of the image forming apparatus has been considerably increased, further increase of the conveyance speed in the post-processing apparatus is nearly facing the limit.
Unexamined Japanese Patent Application Publication No. 2007-156406 (JP2007-156406A) discloses an apparatus which is provided with a superposing section to pile up a plurality of paper sheets in an intermediate conveyance unit, and correctively conveys the piled up paper sheets from superposing section. In this way, by correctively conveying the plurality of piled-up paper sheets, the paper sheet interval can be prolonged according to the number of superposed paper sheets without increasing the conveyance speed. Namely, the productivity in the post-processing section can be improved.
In a post-processing apparatus utilizing the sequential punch processing section disclosed in JP2004-261951A, the drive motor to activate a punch for conducting punch processing, may be the one which can only give a cutting power to form punch holes on a single paper sheet. However, due to the procedure of stopping the paper sheet on the conveyance path for punch processing and conveying the paper sheet again after the punch processing, processing speed, namely moving speed of the punch is required to be increased compared to the case of the batch method.
According to the above background, and various view points such as cost, selections for the configuration of post-processing apparatus having been conducted, as the result, for the sequential punch processing section, in many cases the configuration is selected to be optimal for the maximum thickness of paper sheets (number of paper sheet of the bundled paper sheets) capable of punch processing is a single paper sheet.
In these cases, when the post-processing apparatus having the superposing section disclosed in JP2007-156406A is utilized in combination in order to improve the productivity, a plurality of paper sheets are conveyed in superposed state. In cases where the plurality of paper sheets are punch processed, there can be a possibility of exceeding the upper limit of punch processable thickness. In these cases, excessive load is applied to the punch processing section, and may be caused a failure of forming a right hole or breakage of the punch processing section.
Further, in a light printing area, highly required is to handle various types of paper sheets including very thick paper of extremely high basis weight of paper. In these cases, even in the case of punch processing per single paper sheet, the upper limit of punch processable thickness can be exceed, and problems similar to the above mentioned can be caused.
Considering the above problems, an object of the present invention is to provide a post-processing apparatus with high productivity in total system without causing problems such as the breakage of the punch processing section.